Rapid Progradation and Land Creation by a Major River Subdelta

Many deltas worldwide are experiencing alarming rates of land loss. Successful delta management hinges on knowing both the rates and patterns of land loss, and the capacities of these systems for land gain. We use stratigraphic and geochronologic field data from the ~10,000 km² Lafourche subdelta, Mississippi Delta, USA, to characterize the rates and processes of deposition of a major river-dominated, passive margin delta. The pre-Lafourche coastline is constrained with borehole data; south of this boundary about ~6,000 km² of new land formed between 1.6 and 0.7 ka as the Lafourche subdelta prograded into a shallow bay. This produced a stratigraphic sequence with shelly bayfloor muds, overlain by laminated prodelta silts, then 2-3 m thick mouth bar sands, capped by generally mud-dominated overbank sediments that thin coastward and fine with depth. Optically stimulated luminescence (OSL) ages reveal the timing of mouthbar sand deposition. These show that the trunk channel of the Lafourche subdelta prograded at a rate approaching 150 m/yr and Lafourche created new land at an average rate >5 km²/yr. For comparison, the incipient Wax Lake Delta has prograded at roughly 100 m/yr and created new land at an average rate <2 km²/yr since sedimentation was initiated by dredging of the Wax Lake outlet in 1941. It is notable that the short timescale of measurement of Wax Lake Delta growth makes these measurements susceptible to error. Our data indicate that a fully optimized Mississippi sediment conduit could significantly offset modern land loss rates. Quantifying progradation and land creation rates has valuable implications for coastal restoration through river diversions, and may also give insight into future land growth patterns expected of emergent coastal-delta systems like the Wax Lake Delta.